Chain length effects of phenylene sulfide modifiers on selective acetylene partial hydrogenation over Pd catalysts

We recently reported that polymer chains with phenylene sulfide moieties can efficiently modify the Pd catalyst surface, enabling selective partial hydrogenation of acetylene under ethylene-rich conditions. In the present study, we synthesized oligomeric and polymeric phenylene sulfides with tailored molecular sizes to systematically investigate the catalytic effect of chain length of organic modifiers. The results showed that the increased chain length of the modifiers is beneficial for improving the acetylene partial hydrogenation selectivity by inhibiting adsorption and full hydrogenation of ethylene. This was attributed to the fact that modifiers with a greater number of sulfide groups per molecule can bind more efficiently to the Pd surface for entropic reasons (chelate effect) and thus better inhibit ethylene adsorption. In contrast, the adsorption of acetylene to Pd was nearly unaffected by the presence and size of the modifiers owing to its stronger binding ability and small kinetic diameter. Modifiers with larger chain lengths were also advantageous in improving long-term catalyst stability because of their higher thermochemical stability and greater ability to inhibit the formation of carbonaceous deposits. The present results showed that polymers can act as more efficient and long-lasting selectivity modifiers than commonly used small organic molecules.